Alguém bate à porta.

Bate is the 3rd person singular, present indicative of bater (“to knock / hit”).

• eu bato – I knock
• tu bates – you knock (informal singular)
• ele / ela / você bate – he / she / you knock
• nós batemos – we knock
• vocês / eles / elas batem – you (pl.) / they knock

In Portuguese, the subject pronoun (ele, ela, etc.) is often omitted because the verb ending already shows who is doing the action.
Here, alguém (“someone”) is the subject, so you don’t need ele or ela.

Alguém means “someone / somebody”, and it is grammatically singular.
So the verb must also be singular: alguém bate (“someone knocks”).

If you say batem à porta, the subject is an implicit plural “they”:

• Batem à porta. – “(They) are knocking at the door.” (we don’t know who)

Alguém is an indefinite pronoun meaning “someone / somebody” or sometimes “anyone”:

• Alguém bate à porta. – Someone is knocking at the door.
• Há alguém aí? – Is there anyone there?

A few points:

• It is always singular (no “alguéns” in normal usage).
• It doesn’t show gender; context can clarify if needed.
• In negative sentences, Portuguese usually switches to ninguém (“no one”):
  • Ninguém bate à porta. – No one is knocking at the door.

Both exist, but they usually feel different:

• bater à porta – the standard expression for “to knock at/on the door” (the usual polite knocking).
• bater na porta – more like “to hit / bang the door”, often suggesting force, noise or physical impact.

So:

• Alguém bate à porta. – Someone is knocking at the door.
• Ele bateu na porta com força. – He hit/banged the door hard.

À is a contraction of the preposition a (“to, at”) + the feminine singular article a (“the”):

• a (to/at) + a (the, feminine singular) → à

So à porta literally is “to the door / at the door”.

The grave accent (à) in Portuguese always signals this kind of contraction (a + a or a + as → à / às).

Compare:

• a porta – the door (subject or object)
• à porta – at/to the door (with a preposition)

In context, à porta normally means “at the (main) door here”—it’s understood from the situation which door it is.

You can be more specific:

• Alguém bate à porta de casa. – Someone is knocking at the house door.
• Alguém bate à minha porta. – Someone is knocking at my door.

But in everyday conversation, just à porta is enough when everyone knows which door you’re talking about.

Yes, you can, and it is correct.

• Alguém bate à porta. – Simple present; in European Portuguese it very often means “someone is knocking (right now)”.
• Alguém está a bater à porta. – Present progressive; also “someone is knocking right now”, with a bit more focus on the action being in progress.

In Portugal, the simple present is very commonly used for current, ongoing actions, so Alguém bate à porta is completely natural.

In Brazil, people are much more likely to use the progressive with -ndo and na porta:

• Alguém está batendo na porta. – Someone is knocking on the door.

Alguém bate à porta is grammatically correct in Brazil, but it sounds more formal or literary and less colloquial.

You can change the order, but it sounds unusual or literary in everyday speech.

Natural options:

• Alguém bate à porta. – Neutral, standard.
• Batem à porta. – “They’re knocking at the door” (impersonal “they”).

More marked / stylistic:

• À porta bate alguém. – Puts strong emphasis on “at the door”; sounds poetic or very dramatic.

For normal conversation, keep Alguém bate à porta.

Alguém is a pronoun, not a noun, so it does not take articles:

• Correct: alguém
• Incorrect: um alguém, o alguém (these only occur in rare, playful, or poetic uses)

If you want “a person” with an article, use a noun:

• Uma pessoa bate à porta. – A person is knocking at the door.

Yes, porta is feminine: a porta (“the door”).

Clue:

• Nouns ending in -a are often feminine in Portuguese (with some exceptions).
• The article in the contraction à porta also tells you it’s feminine:
  • a (fem. sg.) → à (a + a)

Other common feminine nouns:

• a casa – the house
• a mesa – the table
• a janela – the window

Approximate European Portuguese pronunciation (broadly):

• alguém → [aɫˈɡɐ̃j]
  
  • dark l [ɫ]
  • nasal vowel/diphthong at the end (-em ≈ nasal “-ei”)
• bate → [ˈbatɨ]
  
  • final e is usually an unstressed [ɨ], like a very reduced “uh”
• à → [a]
  
  • short, open “a”
• porta → [ˈpɔɾtɐ]
  
  • o is open [ɔ], like “o” in British “hot”
  • r between vowels is a tap [ɾ], like Spanish r in cara

Spoken smoothly, it sounds like one rhythm group: [aɫˈɡɐ̃j ˈbatɨ a ˈpɔɾtɐ]
Often the à links closely with porta: [aˈpɔɾtɐ].

Yes, bater à porta is a common fixed expression meaning “to knock at/on the door”.

Bater on its own means “to hit / beat / knock / strike” and appears in many expressions:

• bater em alguém – to hit someone
• bater na mesa – to hit/bang the table
• bater palmas – to clap
• bater à porta de alguém – metaphorical: to approach/contact someone
  
  • O problema bateu à nossa porta. – The problem came knocking at our door.

Use ninguém (“no one, nobody”) instead of alguém:

• Ninguém bate à porta. – No one is knocking at the door.

Notice:

• In Portuguese, negation is usually in the subject or before the verb, not with “anyone” as in English:
  • English: “No one / Nobody is knocking” or “Not anyone is knocking.”
  • Portuguese: Ninguém bate à porta.